Method of producing preform for coupled multi-core fiber, method of producing coupled multi-core fiber, and coupled multi-core fiber

ABSTRACT

Provided is a method of producing a preform  10 P for a coupled multi-core fiber including: an arranging process P 1  for arranging a plurality of core glass bodies  11 R and a clad glass body  12 R in such a way that the plurality of core glass bodies  11 R are surrounded by the clad glass body  12 R; and a collapsing process P 2  for collapsing a gap between the core glass bodies  11 R and the clad glass body  12 R, wherein the respective core glass bodies  11 R have outer regions  16  having a predetermined thickness from the periphery surfaces and made of silica glass undoped with germanium, and the clad glass body  12 R is made of silica glass having a refractive index lower than a refractive index of the outer regions of the core glass bodies  11 R.

TECHNICAL FIELD

The present invention relates to a method of producing a preform for acoupled multi-core fiber and a method of producing a coupled multi-corefiber capable of producing a reliable coupled multi-core fiber, and sucha coupled multi-core fiber.

BACKGROUND ART

An optical fiber used in an optical fiber communication system that iscurrently popular has a structure in which the periphery of one core iscoated by a clad to allow optical signals to propagate through the coreso as to transmit information. In recent years, an amount of informationsignificantly increases along with popularization of optical fibercommunication systems. To handle such an increase of the amount ofinformation to be transmitted, a large number of, that is, tens tohundreds of optical fibers are used in an optical fiber communicationsystem so as to perform large amount and long distance opticalcommunications.

To decrease the number of optical fibers in such an optical fibercommunication system, it is known to use a multi-core fiber in which theperipheries of a plurality of cores are coated by one clad to allowlights to propagate through the respective cores so as to transmit aplurality of signals.

As such multi-core fibers, a non-coupled multi-core fiber and a coupledmulti-core fiber are known. In a non-coupled multi-core fiber,respective cores work as transmission passes independent of each otherand the cores are coupled as weakly as possible. In a coupled multi-corefiber, respective cores are coupled to each other so that the pluralityof cores can be substantially regarded as one multimode transmissionpath. This coupled multi-core fiber enables mode multiplexingtransmission which transmits different signals for respective modes oflights propagating through the cores.

Non Patent Document 1 listed below discloses an example of such acoupled multi-core fiber. According to Non Patent Document 1, the closerthe respective cores of a coupled multi-core fiber are arranged, thestronger the cores are coupled. Therefore, it can be thought that thecores are coupled most strongly when the cores contact with each other.

-   [Non Patent Document 1] Yasuo Kokubun “Novel multi-core fibers for    mode division multiplexing: proposal and design principle” IEICE    Electronics Express, Vol. 6, No. 8

SUMMARY OF INVENTION

Stack-and-draw methods can be examples of a method of producing such acoupled multi-core fiber in which respective cores contact with eachother. In a stack-and-draw method, in a state where rod-shaped coreglass bodies to be respective cores are arranged to be surrounded by aclad glass body to be a clad of an optical fiber, the core glass bodiesand the clad glass body may be drawn while collapsing gaps between therespective glass bodies or in the. Alternatively, in the state where thecore glass bodies and the clad glass body are arranged as describedabove, gaps between the core glass bodies and the clad glass body may becollapsed to produce a preform for an optical fiber, and then thepreform for an optical fiber may be drawn. Thus, in order to produce acoupled multi-core fiber in which respective cores contact with eachother as described above, core glass bodies has to be arranged so as tocontact with each other while their periphery surfaces are exposed.

By the way, cores of an optical fiber are usually made of silica glassdoped with germanium in order to set the refractive index of the coreshigher than that of a clad. This is because silica glass doped withgermanium can prevent loss of light propagating therethrough and thusthe refractive index can be easily set high.

However, germanium existing on the peripheral surface of a core glassbody, that is, on a gas-solid interface volatilizes at a temperaturewhere core glass bodies and a clad glass body are drawn while gapstherebetween are collapsed or at a temperature where gaps are collapsedto produce a preform for an optical fiber due to its characteristic.Therefore, bubbles may be formed around the cores when core glass bodiesare arranged so as to contact with each other while their peripherysurfaces are exposed as described above and then drawing process or acollapse process are performed to produce an optical fiber.

Such bubbles formed around the cores may increase loss of signalspropagating through the cores and may decrease the strength of theoptical fiber, and thus the reliability of the optical fiber may bedecreased.

Therefore, an object of the invention is to provide a method ofproducing a preform for a coupled multi-core fiber and a method ofproducing a coupled multi-core fiber capable of producing a reliablecoupled multicore, and such a coupled multi-core fiber.

In order to achieve the objects, the invention provides a method ofproducing a preform for a coupled multi-core fiber which is providedwith a plurality of cores and in which periphery surfaces of adjacentcores among the cores contact with each other, the method including: anarranging process for arranging a plurality of core glass bodies to bethe plurality of cores and a clad glass body in such a way that theplurality of core glass bodies are surrounded by the clad glass body,and the periphery surfaces of adjacent core glass bodies among the coreglass bodies contact with each other; and a collapsing process forcollapsing a gap between the core glass bodies and the clad glass body,wherein the respective core glass bodies have outer regions having apredetermined thickness from the periphery surfaces and made of silicaglass undoped with germanium, and the clad glass body is made of silicaglass having a refractive index lower than a refractive index of theouter regions of the core glass bodies.

With the method of producing a perform (a base material) for a coupledmulti-core fiber described above, the outer regions of the core glassbodies are undoped with germanium, and thus generation of gas due togermanium volatilizing from the core glass bodies can be prevented evenwhen the core glass bodies are heated in the collapsing process.Therefore, generation of bubbles due to volatilized gas of germanium canbe prevented between the core glass bodies and the clad glass body of aproduced preform for a coupled multi-core fiber. By drawing the preformfor a coupled multi-core fiber produced by the producing methoddescribed above, a reliable coupled multi-core fiber in which generationof bubbles between cores and a clad is prevented can be produced.

A method of producing a coupled multi-core fiber according to theinvention includes a drawing process for drawing the preform for acoupled multi-core fiber produced by the method of producing a preformfor a coupled multi-core fiber described above.

With the method of producing a coupled multi-core fiber described above,the preform for a coupled multi-core fiber in which generation ofbubbles between the core glass bodies and the clad glass body isprevented is drawn, and thus a reliable coupled multi-core fiber inwhich generation of bubbles between cores and a clad is prevented can beproduced.

The invention also provides a method of producing a coupled multi-corefiber which is provided with a plurality of cores and in which peripherysurfaces of adjacent cores among the cores contact with each other, themethod including: an arranging process for arranging a plurality of coreglass bodies to be the plurality of cores and a clad glass body in sucha way that the plurality of core glass bodies are surrounded by the cladglass body, and the periphery surfaces of adjacent core glass bodiesamong the core glass bodies contact with each other; and a drawingprocess for drawing the core glass bodies and the clad glass body whilecollapsing a gap between the core glass bodies and the clad glass body,wherein the respective core glass bodies have outer regions having apredetermined thickness from the periphery surfaces and made of silicaglass undoped with germanium, and the clad glass body is made of silicaglass having a refractive index lower than a refractive index of theouter regions of the core glass bodies.

With the method of producing a coupled multi-core fiber described above,generation of gas due to germanium volatilizing from the core glassbodies can be prevented even when the core glass bodies are heated whiledrawn. Therefore, generation of bubbles due to volatilized gas ofgermanium can be prevented between the cores and the clad of the coupledmulti-core fiber, and thus a reliable coupled multi-core fiber can beproduced.

The outer regions of the core glass bodies may be made of pure silicaglass or silica glass doped with chlorine.

The respective core glass bodies may have inner regions surrounded bythe outer regions and made of silica glass doped with germanium.

The respective core glass bodies may be made of only silica glassundoped with germanium.

When the respective core glass bodies are made of only silica glassundoped with germanium as described above, the respective core glassbodies may be made of only pure silica glass or only silica glass dopedwith chlorine.

A coupled multi-core fiber according to the invention is provided with aplurality of cores and in which periphery surfaces of adjacent coresamong the cores contact each other, wherein the respective cores haveouter regions having a predetermined thickness from the peripherysurfaces and made of silica glass undoped with germanium, and a cladsurrounding the cores is made of silica glass having a refractive indexlower than a refractive index of the outer regions of the cores.

With a coupled multi-core fiber having such a configuration, generationof gas due to volatilized germanium between the cores and the clad canbe prevented at a step of producing the coupled multi-core fiber, andthus generation of bubbles can be prevented between the cores and theclad. Therefore, reduction in yield because of elimination of bubblescan be prevented. Thus, such a coupled multi-core fiber can be reliableand low-cost.

The outer regions may be made of pure silica glass or silica glass dopedwith chlorine.

The respective cores may have inner regions surrounded by the outerregions and made of silica glass doped with germanium.

The respective cores may be made of only silica glass undoped withgermanium. In this case, the respective cores may be made of only puresilica glass or only silica glass doped with chlorine.

As described above, a method of producing a preform for a coupledmulti-core fiber and a method of producing a coupled multi-core fibercapable of producing a reliable coupled multi-core fiber, and such acoupled multi-core fiber can be provided according to the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are views showing an aspect of a coupled multi-corefiber according to a first embodiment of the invention.

FIG. 2 is a sectional view perpendicular to the longitudinal directionof a preform for a coupled multi-core fiber to produce the coupledmulti-core fiber shown in FIGS. 1A and 1B.

FIG. 3 is a flowchart showing processes of a first method of producingthe coupled multi-core fiber shown in FIGS. 1A and 1B.

FIG. 4 is a view showing a state where core glass bodies and a cladglass body are arranged.

FIG. 5 is a view showing an aspect of a drawing process.

FIGS. 6A and 6B are views showing an aspect of a coupled multi-corefiber according to a second embodiment of the invention.

FIG. 7 is a sectional view perpendicular to the longitudinal directionof a preform for a coupled multi-core fiber to produce the coupledmulti-core fiber shown in FIGS. 6A and 6B.

FIG. 8 is a view showing a state where core glass bodies and clad glassbodies are arranged.

EMBODIMENT OF THE INVENTION

Suitable embodiments of a method of producing a preform for a coupledmulti-core fiber, a method of producing a coupled multi-core fiber, anda coupled multi-core fiber according to the invention will be describedhereinafter referring to the drawings. For convenience of understanding,scales of the respective drawings and scales in the followingdescription may differ from each other.

FIRST EMBODIMENT

FIGS. 1A and 1B are views showing an aspect of a coupled multi-corefiber (referred to as a multi-core fiber, hereinafter) according to afirst embodiment of the invention. Specifically, FIG. 1A is a sectionalview perpendicular to the longitudinal direction of the coupledmulti-core fiber according to the first embodiment, and FIG. 1B is aview showing the refractive index distribution along the line B-B ofFIG. 1A.

As shown in FIG. 1A, a multi-core fiber 10 of this embodiment includes:a plurality of cores 11; a clad 12 surrounding the periphery surfaces ofthe plurality of cores 11 without gaps therebetween; an inner protectivelayer 13 coating the periphery surface of the clad 12; and an outerprotective layer 14 coating the periphery surface of the innerprotective layer 13.

In this embodiment, the respective cores 11 are arranged linearly alonga radial direction of the clad 12 while the periphery surfaces ofadjacent cores 11 contact with each other. The respective cores 11 aremade to have substantially the same diameter as each other.

As shown in FIG. 1A, in the multi-core fiber 10, each of the cores 11has a two-layered structure and the respective cores 11 have outerregions 16 having a predetermined thickness from the periphery surfacesof the cores 11 and inner regions 15 surrounded by the outer regions 16.As shown in FIG. 1B, the respective cores 11 have similar refractiveindex distributions to each other, and the refractive index of each ofthe outer regions 16 is set to be constant and higher than therefractive index of the clad 12. The refractive index of each of theinner regions 15 is set to be higher than the refractive index of theouter region 16, and the inner region 15 is made to have the highestrefractive index at the center side thereof and to have substantiallythe same refractive index as the outer region 16 at the periphery sideof the inner region 15. With such configuration, the respective cores 11are made to have a higher refractive index as a whole than the clad 12.

The profile of the refractive index distribution of the inner regions 15can be α power distribution, for example, but is not particularlylimited thereto. The α power distribution referred herein is adistribution having a refractive index n (r) expressed by Equation (1)below.

n(r)=n₁[1−2Δ(r/a)^(α)]^(1/2)  (1)

In the equation, “r” represents a distance from the center of the innerregion 15, “n₁” represents a refractive index of the inner region 15 atthe center thereof, “Δ” represents a relative refractive indexdifference from the outer region 16, and “a” represents an outermostdiameter of the α power distribution. When the refractive indexdistribution of the inner region 15 is set to be the α powerdistribution in whole as set in this embodiment, “a” is the same as thediameter of the inner region 15. The value of “α” can be set to about1.2 to 10, for example, but is not particularly limited thereto andpreferably set to 1.5 to 5 from the viewpoint of preventing breakage ofglass during production. Incidentally, when the inner region 15 is madeto have the α power distribution, the α power distribution may deviatefrom an ideal α power distribution due to unevenly distributed dopantsor the like, however an effective α power distribution as a whole isacceptable.

In the center of the inner region 15, the relative refractive indexdifference from the clad 12 is preferably set to 0.85% to 3.5% and morepreferably set to 1.0% to 1.9% for a core having the refractive indexdistribution of the α power distribution because such a multi-core fiberis more suitable for practical use from the viewpoint of connectioncharacteristics and mode propagation characteristics. In addition, therelative refractive index difference of the outer region 16 from theclad 12 is preferably set to 0.3% to 1.0%.

The inner region 15 having such a refractive index distribution may bemade of silica glass doped with germanium, and the concentration ofgermanium is distributed in such a manner that the refractive indexdistribution shown in FIG. 1B is provided. The outer region 16 is madeof silica glass undoped with germanium. For example, the outer region 16may be made of pure silica glass without any dopant, or may be made ofsilica glass slightly doped with chlorine in order to eliminate OH groupfrom silica at the step of producing the core 11. Such slightly dopedchlorine varies the refractive index of silica glass little. When theouter region 16 is made of pure silica glass or silica glass slightlydoped with chlorine, the clad 12 is made of silica glass doped with adopant that decreases the refractive index. Such a dopant that decreasesrefractive index may be fluorine, for example.

In addition, the diameter of each of the cores 11 of the multi-corefiber 10 can be 5 μm, for example, but is not particularly limitedthereto. The thickness of the outer region 16 may be set to 0.1 μm ormore and more preferably set to 0.2 μm or more. Further, the ratio ofthe diameter of the inner region 15 to the outer diameter of the outerregion 16 is preferably set to 0.5 to 0.97 and more preferably set to0.8 to 0.95 because such a value is practical but is not particularlylimited thereto. Therefore, when the diameter of the core 11 is set to 5μm as described above, the diameter of the inner region 15 can be set to4 μm to 4.8 μm, for example, but is not particularly limited thereto.Thus, the upper limit of the thickness of the outer region 16 should bea thickness ensuring the inner region as described above. Also, thediameter of the clad 12 can be 125 μm, for example, but is not limitedthereto.

In addition, the materials of the inner protective layer 13 and theouter protective layer 14 may be ultraviolet curable resin of differenttypes.

In the multi-core fiber 10 described above, the outer region 16 of eachof the cores 11 has a refractive index between the refractive index ofthe inner region 15 and the refractive index of the clad 12. Therefore,when a light propagates through the core, a low-intensity light canspread farther from the center of the core comparing to a core withoutthe outer region 16. In other word, the profile of the intensitydistribution of a light in the core 11 along a diameter direction has awider base. Therefore, the electromagnetical field distributions oflights propagating through the respective cores 11 adjacent to eachother can have more overlapping parts, and thus the adjacent cores 11can be strongly coupled. In other word, even when the center-to-centerdistances of the cores 11 are large similarly to a plurality of coresarranged separately from each other having the same center-to-centerdistances as the respective cores 11 of the multi-core fiber 10 buthaving no outer regions, a difference of the propagation constantsbetween modes can be larger, and thus the adjacent cores 11 can bestrongly coupled to each other. As described above, the cores adjacentto each other are strongly coupled and the respective cores correlate toeach other so that the cores can be regarded as a multimode transmissionpath as a whole. Therefore, the multi-core fiber 10 described above iscapable of mode multiplexing transmission, in which signals aresuperimposed to respective modes of lights propagating through the cores11. Communications using such mode multiplexing transmission can beMulti-Input Multi-Output (MIMO) communications, for example. Here, sincethe multi-core fiber 10 described above has a big propagation constantdifference between respective modes as described above, the multi-corefiber 10 is capable of mode multiplexing/demultiplexing at input/outputsections of the respective modes more easily comparing to a Graded-Index(GI) multimode fiber and a Step-Index (SI) multimode fiber which arecommon, and thus is more suitable for mode multiplexing transmission. Inaddition, with the multi-core fiber 10, individual lights propagatingthrough the respective cores adjacent to each other can be extractedmore easily comparing to the case where the periphery surfaces of thecores without outer regions contact with each other. When modemultiplexing transmission is performed as described above, it is easierto extract individual lights propagating through the respective coresand thus easier to perform mode separation comparing to the case wherethe periphery surfaces of the cores without the outer regions 16 contactwith each other.

Next, a method of producing the multi-core fiber 10 will be described.

(First Producing Method)

A first method of producing the multi-core fiber 10 will be describedfirst. In the first producing method, a preform for a coupled multi-corefiber (referred to as a preform hereinafter) is produced, and then theproduced preform is drawn to produce a multi-core fiber.

FIG. 2 is a view showing an aspect of a cross-section perpendicular tothe longitudinal direction of a preform to produce the multi-core fibershown in FIGS. 1A and 1B. As shown in FIG. 2, a preform 10P has asubstantially cylindrical shape and includes: a plurality of rod-shapedcore glass bodies 11P to be the respective cores 11; and a clad glassbody 12P that is to be the clad 12 and surrounds the core glass bodies11P. Each of the core glass bodies 11P includes an inner region 15P andan outer region 16P surrounding the inner region 15P. The cross-sectionstructure of this preform 10P is made to be substantially homologouswith the cross-section structure of the multi-core fiber 10 exceptingfor the inner protective layer 13 and the outer protective layer 14. Thepreform 10P described above is drawn and coated as described later to bethe multi-core fiber 10 shown in FIGS. 1A and 1B.

FIG. 3 is a flowchart showing processes for producing the preform 10Pshown in FIG. 2 and the first producing method of the multi-core fibershown in FIGS. 1A and 1B. As shown in FIG. 3, the processes in themethod of producing the preform 10P include: an arranging process P1 forarranging core glass bodies and a clad glass body; and a collapseprocess P2 for collapsing gaps between the core glass bodies and theclad glass body. And the method of producing the multi-core fiber 10further includes a drawing process P3 for drawing the preform 10Pproduced as described above.

<Arranging Process P1>

FIG. 4 is a view showing a state where core glass bodies and clad glassbodies are arranged. In the arranging process P1, firstly, a pluralityof core glass bodies 11R shown in FIG. 4 are prepared. The core glassbodies 11R are glass bodies to be the core glass bodies 11P shown inFIG. 2, and glass bodies finally to be the respective cores 11 of themulti-core fiber 10 shown in FIGS. 1A and 1B. Therefore, the number ofcore glass bodies 11R to be prepared is the same as the number of cores11. In addition, each of the core glass bodies 11R is rod-shaped havingsubstantially the same shape and size as each of the core glass bodies11P shown in FIG. 2. Further, each of the core glass bodies 11Rincludes: an inner region 15R made of a material similar to the materialof the inner region 15 of each of the cores 11; and an outer region 16Rthat surrounds the inner region 15R, is made of a material similar tothe material of the outer region 16 of the core 11, and has apredetermined thickness. Therefore, in this embodiment, the inner region15R is doped with germanium in such a way that the refractive indexthereof is α power distribution. The predetermined thickness of thisouter region 16R is preferably set to 0.1 mm or more and more preferablyset to 0.2 mm or more, but is not particularly limited thereto as longas the inner region 15R can be ensured and germanium doped into theinner region 15R is prevented from volatilizing while heated asdescribed later.

In addition to the preparation of the core glass bodies 11R, clad glassbodies are prepared. Clad glass bodies to be prepared include aplurality of rod-shaped clad glass bodies 12R and one tubular clad glassbody 12T. These clad glass bodies 12R and 12T are glass bodies to be theclad glass body 12P shown in FIG. 2, and glass bodies finally to be theclad 12 of the multi-core fiber 10 shown in FIGS. 1A and 1B. Therefore,the material for the clad glass bodies 12R and 12T is chosen to besimilar to the material for the clad 12 described above.

Next, the plurality of core glass bodies 11R and the plurality of cladglass bodies 12R are arranged inside the through hole of the tubularclad glass body 12T. Specifically, the respective core glass bodies 11Rare arranged in such a way that the plurality of core glass bodies 11Rare in one horizontal line while the periphery surfaces of adjacent coreglass bodies 11R contact with each other, and the respective clad glassbodies 12R are arranged in such a way that the core glass bodies 11R inthe horizontal line are surrounded by the plurality of clad glass bodies12R. Here, it is preferable that the clad glass bodies 12R havingdifferent diameters be prepared and arranged from the viewpoint ofdecreasing gaps inside the through hole of the clad glass body 12Talthough this is not particularly shown.

Thus, a state where the core glass bodies 11R and the clad glass bodies12R and 12T are arranged as shown in FIG. 4 is provided.

<Collapse Process P2>

Next, the arranged core glass bodies 11R and the clad glass bodies 12Rand 12T are heated for collapse. In other word, spaces in the throughhole of the clad glass body 12T such as spaces between the core glassbodies 11R and the clad glass bodies 12R are collapsed so that the coreglass bodies 11R and the clad glass bodies 12R and 12T are integrated.Thus, the core glass bodies 11R become the core glass bodies 11P shownin FIG. 2 with little deformation and the clad glass bodies 12R and 12Tbecome the clad glass body 12P shown in FIG. 2.

Here, when germanium is distributed on the surfaces of the glass bodies,the germanium volatilizes due to its characteristic. However, since thematerial of the outer regions 16R of the predetermined thickness in thecore glass bodies 11R is chosen to be similar to the material of theouter region 16 as described above and thus undoped with germanium.Therefore, generation of gas due to germanium volatilizing from the coreglass bodies 11R is prevented in this process where the core glassbodies 11R are heated. In addition, since the refractive index of theinner region 15P is made to be α power distribution in this embodiment,germanium is doped on the periphery side of the inner region 15P at alow concentration. Therefore, even when the predetermined thickness ofthe outer region 16 is small, generation of gas due to germaniumvolatilization is prevented.

Thus, the preform 10P shown in FIG. 2 is provided.

<Drawing Process P3>

FIG. 5 is a view showing an aspect of the drawing process P3.

As a preparation step for the drawing process P3, the preform 10Pproduced through the arranging process P1 and the collapse process P2 isset in a spinning furnace 110. Then the preform 10P is heated by makinga heating section 111 of the spinning furnace 110 to generate heat. Atthis time, the bottom of the preform 10P is heated to 2000° C., forexample, so as to be melted. Then, glass is melted from the preform 10Pand the glass is drawn. The thus drawn glass which is melted issolidified right after the glass is drawn out of the spinning furnace110 so that the core glass bodies 11P become the cores 11 and the cladglass body 12P becomes the clad 12, whereby a multi-core fiber includingthe plurality of cores 11 and the clad 12 is formed. Thereafter, themulti-core fiber passes through a cooling device 120 so as to be cooledto an appropriate temperature. The temperature of the multi-core fiberwhen it goes into the cooling device 120 is about 1800° C., for example,but the temperature of the multi-core fiber when it comes out from thecooling device 120 is 40° C. to 50° C., for example.

The multi-core fiber that has come out of the cooling device 120 passesthrough a coating device 131 having ultraviolet light curable resin tobe the inner protective layer 13 therein so that the multi-core fiber iscoated with this ultraviolet light curable resin. Then, the multi-corefiber passes through an ultraviolet light irradiating device 132 to beirradiated with an ultraviolet light, whereby the ultraviolet lightcurable resin is cured to form the inner protective layer 13. Next, themulti-core fiber passes through a coating device 133 having ultravioletlight curable resin to be the outer protective layer 14 therein so thatthe multi-core fiber is coated with the ultraviolet light curable resin.Then the multi-core fiber passes through an ultraviolet lightirradiating device 134 to be irradiated with an ultraviolet light,whereby the ultraviolet light curable resin is cured to form the outerprotective layer 14, and thus the multi-core fiber 10 shown in FIGS. 1Aand 1B is provided.

Then, a moving direction of the multi-core fiber 10 is changed by a turnpulley 141 and reeled by a reel 142.

Thus, the multi-core fiber 10 shown in FIGS. 1A and 1B is produced.

As described above, with the method of producing the preform 10Pincluded in the first producing method of this embodiment, the outerregions 16R of the core glass bodies 11R are undoped with germanium, andthus generation of gas due to germanium volatilizing from the core glassbodies 11R can be prevented even when the core glass bodies 11R areheated in the collapse process P2. Therefore, generation of bubbles dueto volatilized gas of germanium can be prevented between the core glassbodies 11P and the clad glass body 12P of the produced preform 10P.

Then, with the method of producing the multi-core fiber 10 using thispreform 10P, the preform 10P in which generation of bubbles is preventedbetween the core glass bodies 11P and the clad glass body 12P is drawn.Therefore, the reliable multi-core fiber 10 in which generation ofbubbles is prevented between the cores 11 and the clad 12 can beproduced.

(Second Producing Method)

Next, a second method of producing the multi-core fiber 10 will bedescribed. The second producing method is different from the firstproducing method in a point that the multi-core fiber 10 is producedwithout producing the preform 10P.

In the second producing method, an arranging process is performedsimilarly to the first producing method. That is, core glass bodies 11Rand clad glass bodies 12R and 12T similar to those of the firstproducing method are prepared and the core glass bodies 11R and the cladglass bodies 12R and 12T are arranged similarly to the first producingmethod. Thus, a state where the core glass bodies 11R and the clad glassbodies 12R and 12T are arranged as shown in FIG. 4 is provided.

Then in the second producing method, the arranged core glass bodies 11Rand clad glass bodies 12R and 12T are set in a spinning furnace whilekeeping their positions with respect to each other. In other word, thecore glass bodies 11R and the clad glass bodies 12R and 12T arranged asshown in FIG. 4 are set in the spinning furnace 110 instead of thepreform 10P shown in FIG. 5.

Then the core glass bodies 11R and the clad glass bodies 12R and 12T areheated by making the heating section 111 of the spinning furnace 110 togenerate heat. With this heat, the core glass bodies 11R and the cladglass bodies 12R and 12T are drawn while collapsing spaces in thethrough hole of the clad glass body 12T such as gaps between the coreglass bodies 11R and the clad glass bodies 12R. At this time, similarlyto the first producing method, the outer regions 16R of thepredetermined thickness in the core glass bodies 11R are undoped withgermanium, and thus generation of gas due to germanium volatilizing fromthe core glass bodies 11R is prevented when the core glass bodies 11Rare heated and melted.

The thus drawn glass which is melted becomes the multi-core fiber 10similarly to the first producing method and then reeled by the reel 142.Thus, the multi-core fiber 10 shown in FIGS. 1A and 1B is produced.

As described above, with the second method of producing the multi-corefiber 10 of this embodiment, generation of gas due to germaniumvolatilizing from the core glass bodies 11R can be prevented even whenthe core glass bodies 11R are heated while drawn. Therefore, generationof bubbles due to volatilized gas of germanium can be prevented betweenthe cores 11 and the clad 12, and thus the reliable multi-core fiber 10can be produced.

SECOND EMBODIMENT

Next, a second embodiment of the present invention will be described indetail referring to FIGS. 6A to 8. Here, components that are identicalor similar to those in the first embodiment are indicated by the samereference numerals and the same explanation will not be repeated unlessotherwise particularly described.

FIGS. 6A and 6B are views showing an aspect of a coupled multi-corefiber (referred to as a multi-core fiber, hereinafter) according to thesecond embodiment of the invention. Specifically, FIG. 6A a sectionalview perpendicular to the longitudinal direction of the multi-core fiberaccording to this embodiment, and FIG. 6B is a view showing therefractive index distribution along the line B-B.

As shown in FIG. 6A, a multi-core fiber 20 of this embodiment includes:a plurality of cores 21; a clad 22 of the shape and the material similarto those of the clad 12 of the multi-core fiber according to the firstembodiment; an inner protective layer 23 and an outer protective layer24 of the shape and the material similar to those of the innerprotective layer 13 and the outer protective layer 14 of the firstembodiment respectively.

The respective cores 21 are arranged linearly along a radial directionof the clad 22 while the periphery surfaces of adjacent cores 21 contactwith each other similarly to the plurality of cores 11 of the firstembodiment. The diameters of the respective cores 21 are set to besubstantially the same as each other.

Each of the cores 21 is different from each of the cores 11 of the firstembodiment in a point that the refractive index thereof is uniform as awhole and the core 21 doesn't have the two-layered structure. As shownin FIG. 6B, the refractive indexes of the cores 21 of this embodimentare set to be higher than the refractive index of the clad 22. The cores21 are made of a material similar to the material of the outer regions16 of the cores 11 of the first embodiment. Which means each of thecores 21 of this embodiment includes an outer region and whole of thecore 21 is made of a material undoped with germanium though the core 21does not particularly have a boundary between the outer region and aninner region. In other word, each of the cores 21 of this embodiment canbe regarded as having an outer region and an inner region made of thesame material and thus having the same refractive index though the core21 does not particularly have a boundary between the outer region andthe inner region.

In the multi-core fiber 20 as described above, the cores 21 contact witheach other, and thus the adjacent cores 21 are strongly coupled and therespective cores 21 correlate to each other so that the cores 21 can beregarded as a multimode transmission path as a whole. Therefore, themulti-core fiber 20 described above is capable of mode multiplexingtransmission, in which signals are superimposed to respective modes oflights propagating through the cores 21 similarly to the multi-corefiber 10 of the first embodiment. However, mode separation in modemultiplexing transmission can be easier when outer regions of cores haverefractive indexes between refractive indexes of inner regions and arefractive index of a clad like the first embodiment.

Next, a method of producing the multi-core fiber 20 will be described.

(First Producing Method)

Also in this embodiment, in a first producing method, a preform for acoupled multi-core fiber (referred to as a preform hereinafter) isproduced, and then the produced preform is drawn to produce a multi-corefiber. Therefore, the first producing method of this embodiment hasprocesses similar to those shown in FIG. 3.

<Arranging Process P1>

FIG. 7 is a view showing an aspect of a cross-section perpendicular tothe longitudinal direction of a preform to produce the multi-core fibershown in FIG. 6A. As shown in FIG. 7, a preform 20P includes: aplurality of rod-shaped core glass bodies 21P to be the respective cores21; and a clad glass body 22P to be the clad 22 surrounding the coreglass bodies 21P. Each of the clad glass bodies 22P is configuredsimilarly to each of the clad glass bodies 12P in the preform 10P of thefirst embodiment. The cross-section structure of the preform 20P is madeto be substantially homologous with the cross-section structure of themulti-core fiber 20 excepting for the inner protective layer 23 and theouter protective layer 24.

Also in this embodiment, an arranging process is firstly performedsimilarly to the arranging process P1 in the first producing methodaccording to the first embodiment. FIG. 8 is a view showing a statewhere core glass bodies and clad glass bodies are arranged. In thisembodiment, the same number of core glass bodies 21R with the number ofthe cores 21 made of a material similar to the material of the cores 21are prepared. Which means core glass bodies 21R, each of which has anouter region having a predetermined thickness and is made of a materialundoped with germanium as a whole though which has no particularboundary between the outer region and an inner region, are prepared.Also, clad glass bodies 22R, 22T similarly to the clad glass bodies 12Rand 12T prepared in the first producing method according to the firstembodiment are prepared.

Then, similarly to the first producing method, the respective core glassbodies 21R are arranged in such away that the plurality of core glassbodies 21R are in one horizontal line in the through hole of the cladglass body 22T while the periphery surfaces of adjacent core glassbodies 21R contact with each other, and the respective clad glass bodies22R are arranged in such a way that the core glass bodies 21R in thehorizontal line are surrounded by the plurality of clad glass bodies22R.

Thus, the core glass bodies 21R and the clad glass bodies 22R and 22Tare arranged as shown in FIG. 8.

<Collapse Process P2>

Next, a collapse process P2 is performed similarly to the firstproducing method according to the first embodiment. With this collapseprocess P2, spaces in the through hole of the clad glass body 22T suchas spaces between the core glass bodies 21R and the clad glass bodies22R are collapsed so that the core glass bodies 21R and the clad glassbodies 22R and 22T are integrated. Thus, the core glass bodies 21Rbecome the core glass body 21P and the clad glass bodies 22R and 22Tbecome the clad glass body 22P, whereby the preform shown in FIG. 7 isprovided.

Here, each of the core glass bodies 21R includes an outer region andwhole of the core glass body 21R is made of a material undoped withgermanium. In other word, each of the core glass bodies 21R has an outerregion and inner region made of the same material undoped with germaniumthough the core glass body 21R does not particularly have a boundarybetween the outer region and the inner region. Therefore, generation ofgas due to germanium volatilizing from the core glass bodies 21R isprevented in this process where the core glass bodies 21R are heated.

<Drawing Process P3>

Next, a drawing process is performed similarly to the first producingmethod according to the first embodiment.

Thus, the multi-core fiber 20 shown in FIGS. 6A and 6B are produced.

As described above, with the method of producing the preform 20Pincluded in the first producing method of this embodiment, each of thecore glass bodies 21R includes an outer region and whole of the coreglass body 21R is undoped with germanium though the core glass body 21Rdoes not particularly have a boundary between the outer region and aninner region. Therefore, generation of bubbles due to volatilized gas ofgermanium can be prevented between the core glass bodies 21P and theclad glass body 22P of the produced preform 20P.

Thus, by drawing the preform 20P, the reliable multi-core fiber 20 inwhich generation of bubbles is prevented between the cores 21 and theclad 22 can be produced.

(Second Producing Method)

Next, a second method of producing the multi-core fiber 20 will bedescribed. In the second producing method of this embodiment, themulti-core fiber 20 is produced without producing the preform 20Psimilarly to the second producing method of the first embodiment.

In the second producing method of this embodiment, an arranging processis performed similarly to the first producing method of this embodiment.That is, core glass bodies 21R and clad glass bodies 22R and 22T similarto those of the first producing method of this embodiment are preparedand the core glass bodies 21R and the clad glass bodies 22R and 22T arearranged similarly to the first producing method. Thus, a state wherethe core glass bodies 21R and the clad glass bodies 22R and 22T arearranged as shown in FIG. 8 is provided.

Next, similarly to the second producing method of the first embodiment,the arranged core glass bodies 21R and clad glass bodies 22R and 22T areset in a spinning furnace 110 to perform a drawing process so that themulti-core fiber 20 shown in FIGS. 6A and 6B is produced.

At this time, similarly to the first producing method of thisembodiment, each of the core glass bodies 21R includes an outer regionand whole of the core glass body 21R is undoped with germanium.Therefore, generation of gas due to germanium volatilizing from the coreglass bodies 21R is prevented when the core glass bodies 21R are heatedand melted.

Therefore, with the method of producing the multi-core fiber 20according to the second producing method of this embodiment, generationof bubbles due to volatilized gas of germanium can be prevented betweenthe cores 21 and the clad 22, and thus the reliable multi-core fiber 20can be produced.

Although the invention has been described above by reference to thefirst and second embodiments as examples, the invention is not limitedthereto.

For example, the material of the outer region 16 of each of the cores 11according to the first embodiment and the material of the core 21according to the second embodiment can be silica glass doped with anydopant as long as the silica glass is undoped with germanium. Forexample, aluminum or phosphorus may be doped as dopant. In this case, anamount of germanium in the inner region 15 of the first embodiment maybe adjusted so as to present the refractive index distribution shown inFIG. 1B. In addition, when the outer region 16 of each of the cores 11according to the first embodiment or each of the cores 21 according tothe second embodiment is doped with aluminum or phosphorus, the clads 12and 22 can be made of pure silica glass or silica glass slightly dopedwith chlorine as long as the refractive index of the clad 12 is lowerthan that of the outer region 16 and the refractive index of the clad 22is lower than that of the core 21.

In the first embodiment, the refractive index distribution of the innerregion is α power distribution, however, the present invention is notlimited thereto. For example, the inner region 15 can present a constantrefractive index distribution with the refractive index higher than thatof the outer region 16. In this case, the material of the inner region15 can be silica glass uniformly doped with germanium, for example. Inaddition, in the first embodiment, the refractive index of the innerregion 15 can be the same as the refractive index of the outer region 16similarly to the second embodiment. The refractive indexes can be thesame by doping aluminum or phosphorus in the outer region 16 andadjusting an amount of germanium doped in the inner region 15, forexample.

In the first and second embodiments, a plurality of cores 11 and 21 arearranged in one horizontal line. However, the present invention is notlimited thereto and cores can be differently arranged. For example, inthe cross-section of a multi-core fiber, a plurality of cores can bearranged in a matrix of three rows and three columns while the peripherysurfaces of adjacent multi-core fibers contact with each other.Alternatively, one core can be arranged at the center of the clad andsix cores can be arranged to surround the one core while contacting withthe one core. Further, the multi-core fibers each having five cores aredescribed in the first and second embodiments as examples, however, eachof the multi-core fibers may have two to four, five or more cores.

Each of the clads 12 and 22 may have a two-layered structure. In thiscase, it is preferable that an inner region of the clad be made ofsilica glass doped with fluorine and an outer region of the clad be madeof pure silica glass or silica glass slightly doped with chlorine, forexample. The reason is that a multi-core fiber as a whole can have ahigh strength by using pure silica glass or silica glass slightly dopedwith chlorine, which has a high strength, for an outer region of a cladsince the breaking strength of the multi-core fiber depends on thestrength of the outer surface of the clad. In this case, the tubularclad glass bodies 12T and 22T in the first and second embodiments may bemade of pure silica glass or silica glass slightly doped with chlorine.

In the embodiments described above, the respective cores 11 are made tohave the same diameters and refractive indexes as each other and therespective cores 21 have the same diameters and refractive indexes aseach other, however, in the present invention, the respective cores 11or 21 may have diameters and refractive indexes different to each other.However, it is preferable that the respective cores 11 have the samediameters and refractive indexes as each other and the respective cores21 have the same diameters and refractive indexes as each other from theviewpoint of coupling the respective cores 11 or 21 strongly. Even whenthe respective cores 11 or 21 have diameters and refractive indexesdifferent to each other, it is preferable that the respective cores bemade to have the same optical wave guiding properties as each other fromthe viewpoint of coupling the respective cores 11 or 21 strongly.

EXAMPLES

Hereinafter, the invention will be more concretely described withexamples and comparative examples, but the invention is not limitedthereto.

First Example

A multi-core fiber was produced similarly to the second producing methodof the second embodiment. Five core glass bodies having the diameter of6 mm and made of silica glass slightly doped with chlorine wereprepared. Silica glass doped with fluorine was used as a material of aplurality of clad glass bodies surrounding the core glass bodies so asto have the relative refractive index difference from outer regions of−0.4%. The core glass bodies and the clad glass bodies described abovewere arranged as shown in FIG. 8 and a drawing process was performed toproduce a multi-core fiber in which the diameter of a clad was about 180μm and the diameter of each of cores was about 8 μm.

In the produced multi-core fiber, no bubbles were particularly foundbetween the cores and the clad.

Second Example

A multi-core fiber was produced similarly to the second producing methodof the first embodiment. Four core glass bodies having the diameter of 5mm were prepared. Each of the core glass bodies was made to have: aninner region having the diameter of 4.85 mm; and an outer region havingthe thickness of 0.15 mm so that the ratio of the thickness of the outerregion to the diameter of the core glass body was about 0.03. The outerregions were made of silica glass slightly doped with chlorine, and theinner regions were made of silica glass doped with germanium withconcentration distribution so that the refractive index was α powerdistribution where α was about 3. The relative refractive indexdifference of the centers of the inner regions (centers of the cores)from the outer regions was set to 1.1%. Silica glass doped with fluorinewas used as a material of the plurality of clad glass bodies surroundingthe core glass bodies so as to have the relative refractive indexdifference from the outer regions of −0.4%. The tubular clad glassbodies were made of pure silica glass.

The core glass bodies and the clad glass bodies described above werearranged as shown in FIG. 4 and a drawing process was performed toproduce a multi-core fiber in which the diameter of the region of theclad where fluorine is doped was about 80 μm, the outermost diameter ofthe clad was 125 μm, and the diameter of each of the cores was about 5μm.

In the produced multi-core fiber, no bubbles were particularly foundbetween the cores and the clad.

Third Example

A multi-core fiber was produced almost similarly to the second example.However, the following points were different from the second example.Each of core glass bodies was made to have the diameter of 2.8 mm, inwhich the diameter of an inner region was 2.6 mm and the thickness of anouter region was 0.2 mm so that the ratio of the thickness of the outerregion to the diameter of a core was about 0.07. The core glass bodieswere made to have the refractive index of α power distribution where αwas about 2.5. The relative refractive index difference of the centersof the inner regions (centers of the cores) from the outer regions wasset to 2.6%. The plurality of clad glass bodies surrounding the coreswere made to have the relative refractive index difference from theouter regions of −0.7%. The tubular clad glass bodies were made of puresilica glass.

Then, a drawing process was performed similarly to the second example toproduce a multi-core fiber in which the diameter of the region of theclad where fluorine is doped was about 80 μm, the outermost diameter ofthe clad was 125 μm, and the diameter of each of the cores was about 4μm.

In the produced multi-core fiber, no bubbles were particularly foundbetween the cores and the clad.

Comparative Example

A multi-core fiber was produced similarly to the first example exceptfor that core glass bodies were made of silica glass including 10 mol %germanium, and a clad glass body was made of silica glass slightly dopedwith chlorine.

In the produced multi-core fiber, a plurality of bubbles were foundbetween cores and a clad thereof. Although it was attempted to cut outapart without bubbles in the longitudinal direction of the producedfiber by eliminating parts having bubbles, no multi-core fiber of 50 mor more continuously having no bubbles could not be obtained. It isassumed that these bubbles were generated because germanium near thesurface of the core glass bodies volatilized.

From the result described above, it was confirmed that when at leastouter regions of core glass bodies to be cores are made of silica glassundoped with germanium, bubbles are not generated between the cores anda clad of a produced multi-core fiber. The reason is assumed thatgeneration of gas due to germanium volatilization was prevented when thecore glass bodies are heated.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, a method ofproducing a preform for a coupled multi-core fiber and a method ofproducing a coupled multi-core fiber capable of producing a reliablecoupled multi-core fiber in which generation of bubbles between coresand a clad is prevented can be provided, and such a coupled multi-corefiber can be provided.

1. A method of producing a preform for a coupled multi-core fiber forproducing a coupled multi-core fiber which is provided with a pluralityof cores and in which periphery surfaces of adjacent cores among thecores contact with each other, the method comprising: arranging aplurality of core glass bodies to be the plurality of cores and a cladglass body in such a way that the plurality of core glass bodies aresurrounded by the clad glass body, and the periphery surfaces ofadjacent core glass bodies among the core glass bodies contact with eachother; and collapsing a gap between the core glass bodies and the cladglass body, wherein the respective core glass bodies have outer regionshaving a predetermined thickness from the periphery surfaces and made ofsilica glass undoped with germanium, and the clad glass body is made ofsilica glass having a refractive index lower than a refractive index ofthe outer regions of the core glass bodies.
 2. The method according toclaim 1, wherein the outer regions of the core glass bodies are made ofpure silica glass or silica glass doped with chlorine.
 3. The methodaccording to claim 1, wherein the respective core glass bodies haveinner regions surrounded by the outer regions and made of silica glassdoped with germanium.
 4. The method according to claim 1, wherein therespective core glass bodies are made of only silica glass undoped withgermanium.
 5. The method according to claim 4, wherein the respectivecore glass bodies are made of only pure silica glass or only silicaglass doped with chlorine.
 6. A method of producing a coupled multi-corefiber comprising drawing the preform for a coupled multi-core fiberproduced by the method of producing a preform for a coupled multi-corefiber according to any one of claims 1 to
 5. 7. A method of producing acoupled multi-core fiber which is provided with a plurality of cores andin which periphery surfaces of adjacent cores among the cores contactwith each other, the method comprising: arranging a plurality of coreglass bodies to be the plurality of cores and a clad glass body in sucha way that the plurality of core glass bodies are surrounded by the cladglass body, and the periphery surfaces of adjacent core glass bodiesamong the core glass bodies contact with each other; and drawing thecore glass bodies and the clad glass body while collapsing a gap betweenthe core glass bodies and the clad glass body, wherein the respectivecore glass bodies have outer regions having a predetermined thicknessfrom the periphery surfaces and made of silica glass undoped withgermanium, and the clad glass body is made of silica glass having arefractive index lower than a refractive index of the outer regions ofthe core glass bodies.
 8. The method according to claim 7, wherein theouter regions of the core glass bodies are made of pure silica glass orsilica glass doped with chlorine.
 9. The method according to claim 7 or8, wherein the respective core glass bodies have inner regionssurrounded by the outer regions and made of silica glass doped withgermanium.
 10. The method according to claim 7, wherein the respectivecore glass bodies are made of only silica glass undoped with germanium.11. The method according to claim 10, wherein the respective core glassbodies are made of only pure silica glass or only silica glass dopedwith chlorine.
 12. A coupled multi-core fiber which is provided with aplurality of cores and in which periphery surfaces of adjacent coresamong the cores contact each other, wherein the respective cores haveouter regions having a predetermined thickness from the peripherysurfaces and made of silica glass undoped with germanium, and a cladsurrounding the cores is made of silica glass having a refractive indexlower than a refractive index of the outer regions of the cores.
 13. Thecoupled multi-core fiber according to claim 12, wherein the outerregions are made of pure silica glass or silica glass doped withchlorine.
 14. The coupled multi-core fiber according to claim 12 or 13,wherein the respective cores have inner regions surrounded by the outerregions and made of silica glass doped with germanium.
 15. The coupledmulti-core fiber according to claim 12, wherein the respective cores aremade of only silica glass undoped with germanium.
 16. The coupledmulti-core fiber according to claim 15, wherein the respective cores aremade of only pure silica glass or only silica glass doped with chlorine.